Over the past several decades the control of air pollution has become a priority concern of society. The United States and other countries have developed elaborate regulatory programs aimed at requiring factories and other major sources of air pollution to install the best available control technology (BACT) for removing contaminants from gaseous effluent streams released into the atmosphere. The standards for air pollution control are becoming increasingly stringent, so that there is a constant demand for ever more effective pollution control technologies. In addition, the operating costs of running pollution control equipment can be substantial, so there is also a constant demand for more efficient technologies.
Air pollution control regulations include regulations for the optical opacity of the effluent gases (see, e.g., "A Changing Environment," Neil Biege, Hank Pretti, and Walt Pirwitz, International Cement Review, p. 73, July 1997). The opacity is often measured in the stack by measuring the obscuration of a light beam passing through the gases. However, most air pollution control regulations define opacity in terms of the visibility of the plume after the water vapor (steam) has condensed out of the plume. One air pollution control problem is the formation of what is commonly described as a "detached plume." The term "detached plume" is commonly used by those skilled in the art to describe an opaque plume that forms or intensifies after leaving the stack and that remains highly visible even after all of the steam has dissipated from the stack emissions. Detached plumes are a particularly intractable problem for cement plants, but are also sometimes associated with other facilities. In many cases, a detached plume is a visible plume that is physically detached from the stack, forming about 10 meters or more from the exit of the stack. However, while the term "detached plume" is physically descriptive of what was observed by many U.S. researchers, this phenomena is also known by other terms. For example, in cold, humid weather conditions, such as those often experienced in Canada, these opaque plumes may appear to originate inside the stacks. These opaque plumes are also known as "secondary plumes," "blue haze," or "reactive plumes," particularly in cold, humid climates such as Canada. However, in these cases the physical phenomena creating the opaque plume is believed to originate from similar chemical mechanisms. Where such a detached plume appears to be "attached" to the stack, its intensity increases a distance from the stack exit. Each of these attached plumes will also be referred to as a "detached" plume.
A detached plume for a cement plant is typically intermittent in nature but once formed a detached plume condition may last for hours or days. Such plumes are highly noticeable. The formation of a visible plume may constitute a regulatory violation and is likely to cause concern from community members near the cement plant. Specifically, detached plumes may violate pollution control regulations for opacity, and are thus a potentially serious problem.
Experimental studies and analysis have identified the detached plumes as being comprised primarily of ammonium sulfate and ammonium chloride particulates that form and condense as emissions from the stack cool in the atmosphere (see, e.g., Larry Edwards and Lee W. Cover, "Simultaneous In-Plume and In-Stack Sampling For Analysis of Detached Plume At A Cement Plant"). The ammonium particulates are in a size range of approximately one micron, which is a size that is particularly efficient at scattering and reflecting light. In cement manufacture, the ammonia in the particulates is generally believed to come from the feed meal used in the cement process itself
Modern cement plants typically use either electrostatic precipitators (ESPs) or fabric filter baghouse (FFB) collectors to remove particulates from the exhaust gas before it is released to the atmosphere. ESPs are capable of high particle collection efficiency of fine particles when the particles have the proper electrical resistivity. FFBs are capable of high particle collection efficiency of fine particles when the particles have the proper characteristics. Typically, pollutant gases are first conditioned such that the particulates can be more efficiently collected by ESPs or FFBs. Typically, the hot cement kiln gases must be cooled to approximately 150.degree. C. to have acceptable emissions from an ESP or cooled to approximately 180.degree. C. to protect a FFB from overheating. Modern energy efficient cement plants typically use two methods to cool and condition hot gases before they enter an ESP or FFB. In a first stage of cooling, a gas conditioning tower (GCT) uses a spray of water to cool and condition the hot gases.
Many modern energy efficient cement plants also perform a second stage of cooling and conditioning subsequent to the GCT by passing the hot cement kiln/preheater gases through the cool wet limestone of the raw feed mill supplying fresh meal to the kiln. This has the beneficial effects of drying the meal and reducing the quantity of cooling water that must be sprayed in the GCT, thereby reducing energy and water consumption in the plant. When the feed mill is on, the gas conditioning tower typically must only cool the effluent to around 250.degree. C., with the remaining cooling of the gases (to approximately 150.degree. C.) occurring in the feed mill. When the feed mill is shut off, the cooling tower must provide all of the cooling, which means that more cooling must occur in the cooling tower. There are thus two distinct operational states of the cement kiln off-gas equipment, corresponding to mill-on and mill-off conditions.
Detached plume formation has been observed to be highly dependent upon the operational state of the cement plant. The formation of detached plumes occurs more frequently (and is typically more prominent) when the raw feed mill is shut off. Since detached plume formation is more frequent in the mill-off state, the inventors believe that the concentration of plume-forming gases in the stacks is substantially higher for the mill-off state.
The pollution control means typically used in modern cement plants is inadequate to remove the chemical constitutes that form detached plumes. Conventional air pollution control techniques previously used for cement plants, such as ESPs and FFBs, do not capture the chemical emissions that are believed to cause the formation of detached plumes.
There are many technical obstacles to eliminating detached plumes from the stacks of cement plants. It is generally known that the release of ammonia is a factor in the formation of detached plumes. However, ESPs and FFBs do not remove gaseous chemicals such as ammonia. In principal, additional chemical scrubbing means could be added subsequent to the particle removal stage to remove the ammonia gases. However, a conventional chemical gas scrubber attached subsequent to an ESP or FFB which could handle the large gas effluent load of a cement plant would be prohibitively expensive and consume large quantities of energy and large volumes of chemicals. Additionally, it may be possible to abate detached plumes by reducing the concentration of ammonia and other plume-forming gases in the source materials such that the concentration of ammonia in stack emissions is substantially reduced. However, the reduction or elimination of potential gas phase reaction components from raw materials (e.g., ammonia, chlorides, and sulfur in raw feed meal), has been largely unsuccessful because of the practical problems associated with substantially reducing the concentration of ammonia and other plume-forming chemicals from source materials. (See, e.g., Ronald L. Hawkes and Thomas Rose, "A Proactive Approach to Minimizing Opacity From Cement Kilns," 1995 IEEE Cement Industry Technical Conference, Institute of Electrical Engineers, Inc. (1995), pp. 451 et seq.).
No known conventional pollution control provides an economical, efficient, and reliable method to remove the gases which may create detached plumes. No known conventional pollution control system for cement plants efficiently prevents the formation of detached plumes.
What is desired is a practical method to prevent the formation of detached plumes.